In an Orthogonal Frequency Division Multiple Access-Time Division Multiple Access (OFDMA-TDMA) scheme, radio resources are divided into planar time-frequency grids. Each grid represents a certain sub-carrier in a certain time interval. Each access user transmits and receives signals in an allocated waveband and time slot. Due to dispersion characteristics of radio channels, e.g. fading, multipath and interference, receiving qualities of information bits transmitted on different sub-carriers and in different time intervals may be different. Therefore, a part of the information bits passing through such radio channels may be lost because of the serious decay of channel receiving quantities.
A conventional strong channel coding scheme can be used to recover damaged bits. An improved channel coding scheme, e.g. Turbo code and LDPC code, is a most effective scheme when a code block is long enough, about thousands of bits and a coding rate is between a low rate and a middle rate. However, a simple channel coding scheme has no advantages when the code block is relatively short and the coding rate is relatively high. In addition, the improved coding scheme results in high complexity.
Therefore, embodiments of the present invention provide a time-frequency code spreading method and apparatus to combat time-frequency selectivity of channels in the OFDMA system, in which a code spreading technology based on each sub-channel is applied to the OFDMA system. In the embodiments of the present invention, advantages of the code spreading technology are not limited by bandwidth, so as to combat the time-frequency selectivity of channels and interference.